Motion compensated multi-wavelength digital holography

ABSTRACT

A holography imaging system includes a first laser, a second laser, a transmitter optical system, a receiver optical system, and a detector array. The first laser has a constant frequency, and the second laser has a non-constant frequency. The transmitter optical system can illuminate a target simultaneously using portions of the first and second laser signals. The receiver optical system can focus a returned light onto the detector array. A first and second illumination point sources can direct portions of the first and second laser signals onto the detector array. The first and second illumination point sources are located in-plane with a pupil of the receiver optical system. The system can detect simultaneously holograms formed on the detector array based on the returned light and the portions of the first and second laser signals directed by the first and second illumination point sources.

FIELD

The disclosure relates in general to laser radar three-dimensionalimaging and synthetic aperture ladar and relates, in particular to, forexample, without limitation, holography imaging systems and methods.

BACKGROUND

In the field of coherent laser radar, target motion or vibrationpresents a considerable challenge to coherent combination of data over along dwell time. Target motion can cause speckle decorrelation of thereflected light within short time scales (100 nanoseconds (ns)—a fewmicroseconds (μs)). In some applications, it is desirable to coherentlycombine data over much longer time scales (10's of miliseconds (ms) orlonger).

The description provided in the background section should not be assumedto be prior art merely because it is mentioned in or associated with thebackground section. The background section may include information thatdescribes one or more aspects of the subject technology.

SUMMARY

One or more implementations of the subject disclosure are illustrated byand/or described in connection with one or more figures and are setforth in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a schematic diagram of an exemplary embodiment ofthe subject technology.

FIG. 1B illustrates a schematic diagram of another exemplary embodimentof the subject technology.

FIG. 2 illustrates an example of the lateral geometric arrangement forthe placement of the two local oscillator point sources relative to theexit pupil of a receiver optical system.

FIG. 3 illustrates an example of the location in the Fourier domain ofthe complex-valued pupil plane field data for wavelengths A and B.

FIG. 4 illustrates an example of a top-level view of the time dependentfrequency of the transmitted pulse sequence, and an example of atop-level view of the processing steps of conjugate product and FFT toproduce a 3D image.

FIG. 5 illustrates an example of a diagram showing the processing stepsto produce a 3D image of an illuminated target.

FIG. 6 is an example of a diagram describing the steps in the globalphase estimation operation.

FIG. 7 illustrates an example of a photograph of a miniature model usedas a test target.

FIG. 8 illustrates an example of a range image and point cloud displayof 3D imagery collected from the test target using the subjecttechnology, demonstrating unambiguous high precision ranging.

FIG. 9 illustrates an example of a range profile in a single pixel ofthe 3D data, demonstrating the range resolution capability.

FIG. 10 illustrates an example of the offset frequency configuration forsynthetic wavelength synthetic aperture ladar operation.

FIG. 11 illustrates an example of a diagram describing the dataprocessing steps for synthetic aperture ladar operation.

FIG. 12 is a block diagram illustrating an example of a computer systemwith which one or more configurations of the subject technology may beimplemented.

DETAILED DESCRIPTION

The detailed description set forth below is intended as a description ofvarious configurations of the subject technology and is not intended torepresent the only configurations in which the subject technology may bepracticed. As will be realized, the subject technology is capable ofother and different configurations and its several details are capableof modification in various other respects, all without departing fromthe scope of the subject technology.

One or more aspects of the subject technology relate to laser radar 3Dimaging and synthetic aperture ladar, and, in particular, for example,without limitation, relate to motion compensated multi-wavelengthdigital holography. One or more aspects of the subject technology relateto motion compensated multi-wavelength digital holography for 3D imagingand synthetic aperture ladar.

In one or more implementations, the subject technology provides a motioncompensated digital holography system that solves a speckledecorrelation problem (e.g., the speckle decorrelation problem arisingfrom target motion and/or vibration in coherent laser radars), allowingmulti-wavelength coherent data combination over long time scales. Thedisclosure below provides various approaches of digital holography anddiscusses two applications where long duration coherent data combinationis desirable and enabled by the subject technology: 3D imaging andsynthetic aperture ladar. In addition, the subject technology iscompared to the various alternative approaches. Furthermore, thedisclosure provides examples of various implementations of the subjecttechnology.

Various Approaches

Digital holography is a method of coherent laser radar (ladar) where theobject or target is flood-illuminated with a laser signal and thereflected light (or the returned laser signal) from the target isdetected by an array of detectors located in an image plane of anoptical system. The reflected light is interfered with one or moreoff-axis reference beams (local oscillators) to form holograms on thefocal plane array. The recorded holograms have interference fringes withan orientation corresponding to the pupil plane lateral geometry of thelocal oscillators relative to the receiver aperture.

Computing a two-dimensional (2D) Fourier transformation on the recordeddata produces complex-valued pupil plane field data. Image planecomplex-valued data can then be computed by inverse Fouriertransformation of the pupil plane field data after appropriate spatialfiltering. Many advanced capabilities are enabled by this method ofdetecting both the intensity and phase of the returned light. Forexample, aberrations caused by imperfect optics or refractive turbulencein the atmosphere may be computed and removed digitally from thecoherent imagery using image sharpness maximization.

Digital holography may be used to produce three-dimensional (3D) imagesof an object by operating at two wavelengths and computing the phasedifference between coherent images recorded at each wavelength. Thismethod is referred to as dual-wavelength 3D imaging. There are threelimitations of dual-wavelength 3D imaging that are relevant incomparison to the subject technology, which are discussed below. First,there is range ambiguity in dual-wavelength 3D imaging. The ambiguityinterval, R_(amb), is related to the frequency difference of the twowavelengths, Δf, by

$\begin{matrix}{R_{amb} = \frac{c}{2\Delta\; f}} & (1)\end{matrix}$where c is the speed of light. This ambiguity causes phase wraps acrossan image. Two dimensional phase unwrapping algorithms may be applied tocombat phase wrapping, but these algorithms can fail for objects thathave range discontinuities. An example of a range discontinuity is therange discontinuity between the roof of a building and the ground, whenviewed from an airborne or space based platform.

The ambiguity interval of a dual-wavelength system may be increased toreduce the phase wrapping problem, but only at the expense of rangeprecision since range precision, c_(R), scales linearly with theambiguity interval. The equation relating the two comes from theCramer-Rao lower bound,

$\begin{matrix}{\sigma_{R} = \frac{R_{amb}}{2\pi\sqrt{N_{avg}{CNR}}}} & (2)\end{matrix}$where N_(avg) is the number of incoherent averages and CNR is thecarrier-to-noise ratio.

The second limitation of dual-wavelength 3D imaging is that it providesno range resolution. 3D information in each pixel is computed bycalculating a difference of the phase from each wavelength. The resultis a single value for range in each pixel. If the pixel field of viewcontains multiple down range targets, the phase difference calculationwill produce a range estimate that combines the data from all down rangetargets.

The third limitation of dual-wavelength 3D imaging is that data canusually not be coherently combined from frame to frame due to targetmotion and/or vibration.

In another approach, a motion compensated frequency-modulatedcontinuous-wave (FMCW) system may solve the multi-frame coherent datacombination problem for 3D imaging in a different way than the subjecttechnology. The FMCW system employs temporal heterodyne detection whereone or more configurations of the subject technology employ spatialheterodyne detection. The FMCW approach does not have a way to separatethe chirped laser signal from the pilot reference laser signal, so anonlinear operation is required to put power into the frequencydifference of the two heterodyne beat signals.

The motion compensated FMCW approach has two key limitations. First, thenonlinear operation required in the FMCW method creates significantexcess noise due to self-mixing of shot noise. This places much highpower requirements on the laser transmitter to overcome the excessnoise. Second, to make the FMCW approach applicable to realistic degreesof target motion, the nonlinear operation should occur within the cameraprior to digitization. This means that custom camera electronicsdevelopment is required. The above discussion describes multi-frame 3Dimaging which requires long duration coherent data combination.

A second application where long duration coherent data combination isrequired is synthetic aperture ladar. Synthetic aperture ladar is amethod to take advantage of the translation of a moving platform to forma synthetic aperture with length exceeding the size of the real apertureby coherently combining data collected while the synthetic aperture istraversed. This approach allows imaging a target with cross rangeresolution exceeding the diffraction limit of the real aperture. Sincethe data is collected sequentially in time while the aperturetranslates, long duration coherent data combination is required. For atypical airborne imaging scenario, the aperture translation time is 10'sof ms, which is far longer than typical vibrating or moving targetscattered light coherence times, which are approximately 100 ns to a fewμs. Thus, target motion presents a significant challenge to syntheticaperture ladar. In particular, uncorrelated space-variant motion on theorder of the wavelength will ruin cross range compression.

There are synthetic aperture radar (SAR) algorithms to correct forvarious kinds of correlated motion, for example, subpatch processing forrigid body motion and space-variant refocus for bending and flexing.However, moving or vibrating targets will likely have significantuncorrelated components to the motion relative to the small scale ofoptical wavelengths. Examples of vibrating targets include tanks orvehicles with the engine running or building walls vibrating from aheating, ventilation, and air-conditioning (HVAC) system. The SAR ruleof thumb for uncompensated wide band phase error is 0.07 radians rootmean square (RMS). For a 10 GHz SAR system, this rule of thumb indicatesthat allowable uncompensated motion is 170 μm RMS. By comparison, for asynthetic aperture ladar system operating at a 200 THz carrierfrequency, allowable motion is only 8.4 nm RMS.

Examples of Aspects and Advantages of Subject Technology

In one or more implementations, the subject technology provides a motioncompensated digital holography system that addresses the speckledecorrelation problems arising from the target motion and/or vibrationin coherent laser radars. The motion compensated digital holographysystem allows multi-wavelength coherent data combination over long timescales.

In one or more implementations, the subject technology overcomes variouslimitations of the dual-wavelength 3D imaging discussed above. First,while dual-wavelength 3D imaging produces range ambiguity, in one ormore aspects, the subject technology avoids range ambiguity whileproviding fine range precision. Second, while dual-wavelength 3D imagingprovides no range resolution, in one or more aspects, the subjecttechnology can provide a range resolving capability by utilizing aplurality of wavelengths. A system of the subject technology can providemultiple range reports within a single pixel field of view. The thirdlimitation of dual-wavelength 3D imaging is that data can usually not becoherently combined from frame to frame due to target motion. Bycontrast, in one or more aspects, the subject technology enablescoherent data combination across frames, boosting the waveform CNR thatallows operation at significantly lower laser power for equal rangeprecision performance.

According to one or more aspects, the subject technology can achievethese advantages in 3D imaging, over the dual-wavelength 3D imagingapproach described above, by operating at a plurality of wavelengthsover multiple frames. Multi-frame operation in the 3D imagingapplication is why long duration coherent data combination is desirable.The total dwell duration is determined by the camera frame rate and thenumber of frames desired. High-speed digital camera frame rates are near1 kHz, depending on the array size. It is often desirable to coherentlyprocess 10's of frames, leading to a total dwell time of 10's ofmiliseconds. This is much longer than typical moving target scatteredlight coherence times, which are approximately 100 ns to a few μs.

It is conceptually possible to perform multi-wavelength imaging in asingle frame (short duration) by employing angular multiplexing of manylocal oscillators. This approach has two significant limitations. First,simultaneously collecting N wavelengths carries a shot noise penalty ofN, reducing the CNR by a factor of N. Furthermore, for a masteroscillator power amplifier source architecture, where all N wavelengthsare amplified by a single common optical amplifier, there is anadditional CNR penalty of N due to sharing of available power among theN wavelengths, resulting in a N² penalty. For values of N greater than10, this CNR penalty quickly becomes too large. The second significantlimitation is that angular multiplexing with many wavelengths requiresvery high spatial oversampling, which limits the field of view.

For an imaging configuration in which the target, sensor and atmosphereare highly static, multi-frame coherent combination can be achievedwithout motion compensation. However, a sufficiently static arrangementcan only be achieved in the laboratory. To be sufficiently static, themagnitude of target motion, including bending, flexing, or vibrating,must be much less than the laser wavelength. Furthermore, target rigidbody rotation must be less than D/R, where D is the receiver aperturediameter and R is the range to the target. For airborne imagingapplications, this could be as low as a few μ rads. Therefore, anyapplication outside of the laboratory could not employ opticalmulti-frame coherent data combination without motion compensation.

In one or more aspects, the subject technology differs from the FMCWapproach described above in many beneficial ways. While the FMCWapproach does not have a way to separate the chirped laser signal fromthe pilot reference laser signal, in one or more aspects, the subjecttechnology employs angular multiplexing of the local oscillators toseparate the data in the spatial frequency domain, enabling astraightforward phase reference as discussed in greater detail below.Furthermore, unlike the FMCW approach, which creates significant excessnoise due to self-mixing of shot noise, in one or more aspects, thesubject technology does not have this excess noise problem due to thereceiver angular multiplexing. Moreover, while the FMCW approachrequires custom camera electronics development, in one or more aspects,the subject technology does not require such custom development. In oneor more aspects, because the subject technology does not require thenonlinear operation, it is compatible with currently availablecommercial cameras without any need for customization or modification.

In addition, in one or more aspects, the subject technology can solvethe synthetic aperture ladar target motion sensitivity problem,discussed above as relating to the synthetic aperture ladar method, byemploying a radio frequency (RF) offset between the chirped and thereference laser signals as discussed in more detail below. The systemwould effectively operate at a synthetic wavelength, reducingsensitivity to target motion at the expense of increasing the datacollection time.

Various Implementations of Subject Technology

FIG. 1 illustrates a schematic diagram of an example of a hardwareimplementation of the subject technology. In this example, a digitalholography imaging system (100) employs two lasers (101 and 102), wherethe frequency of the laser signal of laser A (101) is held constant, andthe frequency of the laser signal of laser B (102) is linearly tuned, orchirped during data collection. In this example, the frequency of thelaser signal of laser A (101) is constant over time, where the frequencyof the laser signal of laser B (102) varies over time (or non-constantover time). The constant frequency laser signal (namely, the lasersignal of laser A (101)) serves as a phase reference signal for thecoherent data combination. The linearity of the frequency tuning of thelaser signal of laser B (102) can be ensured by use of a frequencycontrol servo (103). The frequency control servo may detect a delayedself-heterodyne signal and lock the signal to an RF reference signalthat determines the desired chirp rate. The frequency control servo(103) is not needed in all implementations of the subject technologysince the global phase compensation calculation described below canremove the effect of some frequency chirp nonlinearity.

According to one or more aspects, the subject technology may also beimplemented with stepped frequency changes in the frequency of the lasersignal of laser B (102) instead of linear tuning described above. Ineither case, the outputs of laser A and laser B are transmitted to atarget (120) through a transmitter optical system (104), and may becombined (105) using a combiner (not shown) and/or amplified using anamplifier (106) before being transmitted. The target isflood-illuminated (107), and scattered light (108) from the target isreturned and received by imaging optics (110) having one or more lens.The returned light is focused onto a focal plane array (FPA) (109).

Part of each laser signal of laser A (101) and laser B (102) is splitoff (112 and 111) so that such part of each laser signal can serve as alocal oscillator. The local oscillators are directed (113 and 114)through optical fibers (123 and 124) toward the focal plane array (109).The digital holography imaging system (100) may also include a computersystem (140) to receive and process images from the focal plane array.

In one or more implementations, a focal plane array may include acamera. In one or more implementations, the amplifier (106) may includeone or more amplifiers. In one implementation, a combiner may be locatedafter the lasers A and B but before the amplifier (106). In anotherimplementation, a first amplifier may be located after laser A, a secondamplifier may be located after laser B, and a combiner may be locatedafter the first and second amplifiers. In one or more implementations, adetector array is a focal plane array.

FIG. 1B illustrates a schematic diagram of another exemplary embodimentof the subject technology. A digital holography imaging system shown inFIG. 1B is the same as the digital holography imaging system shown inFIG. 1A, except that the digital holography imaging system shown in FIG.1B includes one or more amplitude modulators for pulsed mode operation.Local oscillator light from laser A (101) is modulated into a shortpulse by an amplitude modulator (AM) (151). Likewise, local oscillatorlight from laser B (102) is modulated into a short pulse by anotheramplitude modulator (152). The transmitted light is modulated aftercombining lasers A and B at a combiner (105) (not shown) by an amplitudemodulator (153). A timing control system (154) controls the timing ofthe amplitude modulators to cause temporal overlap of the returnedsignal pulse (108) with both of the local oscillator pulses (113 and114).

The position of the output fiber tips (133 and 134) of the opticalfibers (123 and 124) are in-plane with the exit pupil of the imagingoptics (110) of the receiver optical system and arranged according tothe lateral geometry shown in FIG. 2. FIG. 2 illustrates the geometrywith a pupil plane coordinate system where the exit pupil (202) iscentered at the origin, LO A (204) is located at (x,y) coordinates

$\left( {{- \frac{QD}{4}},{- \frac{QD}{4}}} \right),$and LO B (206) is located at coordinates

$\left( {\frac{QD}{4},{- \frac{QD}{4}}} \right),$where D is the exit pupil diameter, and Q is the sampling ratio, definedas

$\begin{matrix}{{Q = \frac{z_{i}\lambda}{Dp}},} & (3)\end{matrix}$where z_(i) is the distance between the exit pupil and the focal planearray, λ is the laser wavelength, and p is the pixel pitch of the focalplane array. For LO A, λ is the wavelength of the laser A. For LO B, λis the wavelength of the laser B. Thus, the output fiber tips (133 and134) for local oscillators LO A and LO B (204 and 206) serve as twopoint source illuminators (or two illumination point sources) in thepupil plane of the receiver optical system.

In one or more aspects, the returned light associated with the lasersignal of laser A will interfere with local oscillator A (204) to form afirst hologram A on the FPA. Likewise, the returned light associatedwith the laser signal of laser B will interfere with local oscillator B(206) to form a second hologram B superimposed on the FPA. Targetillumination and detection at the two wavelengths occur simultaneously.The recorded frame is a superposition of the first hologram A and thesecond hologram B.

In one or more implementations, simultaneous detection is accomplishedby the angular multiplexing of the two wavelengths that enablesimultaneous recording of coherent images at each wavelength, asexplained in more detail below. In one or more implementations,simultaneous detection is the same as simultaneous illumination of thefocal plane array. In one or more implementations, simultaneousillumination can be accomplished by illuminating the focal plane arraysimultaneously using the returned light and the local oscillators (LO Aand LO B). In one or more implementations, the returned light may be areturned pulse, and the two local oscillators can be two pulses. In oneor more implementations, simultaneous illumination can be accomplishedby superimposing and forming the first hologram A and the secondhologram B on the focal plane array simultaneously.

The 2D Fourier transform of the recorded data appears as illustrated inFIG. 3, according to one or more aspects of the subject technology. Thelaser signal from laser A has wavelength A that is constant, and thelaser signal from laser B has wavelength B that varies over time. Pupilplane complex-valued field data from laser A appears in the upper right(304) and lower left quadrants (308) in the spatial frequency domain,where one (304) is the conjugate twin of the other (308). Likewise,pupil plane complex-valued field data from laser B appears in the upperleft (302) and lower right quadrants (306), where one (302) is theconjugate twin of the other (306).

Since laser A (101) and laser B (102) operate at different carrierfrequencies, there will not be appreciable cross mixing of hologram Aand hologram B present in the recorded frame. Furthermore, the spatialfrequency domain will contain a large DC component which is the spatialautocorrelation function of each local oscillator (204/206) as well asthe autocorrelation of the received signal (or returned light) which iscone shaped and centered at the origin. By making the irradiance of thelocal oscillators (204 and 206) much brighter than the received light(or returned light), the amplitude of the signal autocorrelationcomponent becomes small and negligible. In one or more aspects, bycropping the Fourier transform to contain only one set of pupil A (304)data (not both 304 and 308) and performing inverse Fourier transform, acoherent image at wavelength A is formed. Likewise, in one or moreaspects, a coherent image at wavelength B can be formed using only datain the pupil B (302) region (not both 302 and 306). This angularmultiplexing of the two wavelengths (A and B) enables simultaneousrecording of coherent images at each wavelength.

FIG. 4 illustrates an example of a top-level view of the time dependentfrequency of the transmitted pulse sequence (402), and a top-level viewof the processing steps of conjugate product and Fast Fourier Transform(FFT) to produce a 3D image, according to one or more aspects of thesubject technology. A 3D image (406) is created by recording a sequenceof angularly multiplexed holograms (404) where the frequency of thelaser signal of laser B is tuned linearly and the frequency of the lasersignal of laser A is held constant. In this example, the transmittedpulse sequence (402) includes a constant laser pulse sequence (402A)from laser A, and a linearly varying laser pulse sequence (402B) fromlaser B. Equivalently, the frequencies of the laser signal of laser Aand the laser signal of laser B may both be tuned as long as thefrequency difference Δf_(AB) between them is linearly chirped, whereΔf_(AB)=f_(A)−f_(B) and f_(A) is the carrier frequency of laser A andf_(B) is the carrier frequency of laser B. In either case, two coherentimages are formed for each frame of recorded data, one at wavelength Aand the other at wavelength B.

According to one or more aspects of the subject technology, a phasedifference image is then computed by taking the conjugate product of thetwo image plane data sets. This conjugate product image is denoted as E.This image E is equivalent to an image formed by the dual-wavelengthdigital holography method discussed above. This process is performed foreach of the holograms in the stack of recorded frames. The result is astack of conjugate product images (404) where the frequency offset ofeach is different. By performing a Fourier transform through the stackof conjugate product images, a 3D image (406) is formed.

This imaging approach compensates for target motion. There are two kindsof motion compensation present in this system: intra-pulse andinter-pulse. Intra-pulse target motion (or motion which occurs duringthe laser pulse or the FPA frame integration time), is addressed bysimultaneous data collection at the two wavelengths. This preventsmotion-induced speckle decorrelation between the data at the twowavelengths. If, alternatively, data at the two wavelengths wascollected sequentially in time, motion-induced decorrelation couldoccur. Too much motion during the frame integration time may cause aloss of fringe visibility resulting in an efficiency penalty. Theefficiency penalty may be reduced by using a pulsed laser system wherethe pulse duration is shorter than the coherence time of the returnedlight reflected off the target.

On the other hand, inter-pulse target motion is motion that occursbetween frames. Compensating for this inter-pulse motion is the functionof laser A, the constant frequency laser. For each recorded frame, thewavelength A coherent image provides a pixel-by-pixel phase referencefor the wavelength B image, since motion alters the phase in each pixelof images A and B in nearly the same way. The conjugate productoperation, producing the E image, shifts the phase in each pixel ofimage B by the negative of the amount measured at wavelength A, therebyremoving any random phase change produced by target motion inducedspeckle decorrelation. In one or more implementations, thispixel-by-pixel motion compensation is a key element.

By sensing the target at a plurality of wavelengths, one or more aspectsof the subject technology provide two key features: (1) high precisionranging with large ambiguity intervals and (2) range resolution. Therange ambiguity is determined by the carrier frequency step size betweenadjacent wavelength measurements, Δf, according to

$\begin{matrix}{R_{amb} = \frac{c}{2\Delta\; f}} & (4)\end{matrix}$where c is the speed of light. By setting the range ambiguity to belarger than the target depth, range ambiguity in the resulting image iseliminated. The range resolution is determined by the total bandwidthspanned by the N equally spaced wavelengths according to

$\begin{matrix}{{\Delta\; R} = {\frac{c}{2\left( {N - 1} \right)\Delta\; f}.}} & (5)\end{matrix}$

For dual wavelength operation (N=2), R_(amb)=ΔR, which results in noresolution capability. For multi-wavelength operation with the subjecttechnology, the range resolution element is smaller than the ambiguityinterval. The number of range resolution elements within an ambiguityinterval is

$\begin{matrix}{\frac{R_{amb}}{\Delta\; R} = {N - 1.}} & (6)\end{matrix}$

FIG. 5 illustrates a diagram showing an example of a processing sequenceto produce a 3D image of an illuminated target, according to one or moreaspects of the subject technology. First at the top left is anillustration of the stack of recorded holograms (502). For eachhologram, the 2D Fourier transform (504) is computed using the FFTalgorithm. By appropriate data cropping in the Fourier domain (506), astack of coherent images is formed corresponding to laser A (508) andlaser B (510). At this point in the processing, aberration correctionmay be performed to remove aberrations from imperfect optics orrefractive turbulence using an image sharpness maximization method.

In some aspects, for each value of the wavelength separation, aconjugate product image is formed, which is denoted as E (512). Therewill likely be a random global phase shift on each of the conjugateproduct images due to radial target translation, frequency chirpnonlinearity, or local oscillator phase drift. This random global phaseis removed using a global phase compensation operation (514), which isdescribed in detail later with reference to FIG. 6. The operation usesstandard nonlinear optimization routines to maximize the sharpness ofthe resulting 3D image (516).

After global phase compensation is completed (514), a cube of 3D data(516) is produced by one-dimensional (1D) Fourier transformation throughthe image stack for each pixel. Next, the peak range report for eachpixel is computed with high precision by Fourier domain zero padding toproduce interpolated points in the range dimension followed by parabolicpeak estimation (518). A parabola is formed using three points at thepeak and the location of the summit of the parabola is reported. A peakselection algorithm (520) may also be employed that seeks to rejectnoise reports by comparing the range report in each pixel to theneighboring pixels. If the value of the range report in a pixel exceedsthe median of the neighborhood by more than the standard deviation ofthe neighborhood, the algorithm selects the next highest peak and thetest is repeated. The final output is a range image (522).

FIG. 6 is a diagram describing an example of the steps in the globalphase compensation operation (514) of FIG. 5, according to one or moreaspects of the subject technology. A guess for the global phase errorfor each of the images, φ_(n) (604), is applied to each pixel of then-th conjugate product image, E_(n) (602), followed by FourierTransformation to create a 3D image (606). The 3D intensity image (I)(610) is computed by magnitude squared (608) of the complex-valued 3Dimage (606). The sharpness (614) is computed by I raised to an exponentβ (usually β=1.2) and summing over all 3 dimensions (612). This producesa measure of the resulting sharpness (614) which the iterativeoptimization algorithm seeks to maximize.

FIG. 7 illustrates an example of a photograph of a miniature model usedas a test target (702), according to one or more aspects of the subjecttechnology. The subject technology has been reduced to practice anddemonstrated. A miniature test target (702) was constructed containingmany range discontinuities. The target scene includes miniature modeltrees, buildings and vehicles.

FIG. 8 illustrates an example of a range image (802) and point cloud(804) display of 3D imagery collected from the test target (702),demonstrating unambiguous high precision ranging, according to one ormore aspects of the subject technology. The test target (702) was imagedutilizing the subject technology. To demonstrate the motioncompensation, the target was rotated 1 mrad between each collectedframe, which is enough to fully decorrelate the speckle pattern sinceD/R=6 mm/3 m=1 mead. The resulting range image (802) and 3D point cloudimage (804) demonstrate that the subject technology works and is able toimage complex targets without range ambiguity.

FIG. 9 illustrates an example of a range profile in a single pixel ofthe 3D data demonstrating the range resolution capability, according toone or more aspects of the subject technology. This figure shows therange resolved return for a single pixel (902) which contains returnfrom a tree branch and roof top.

FIG. 10 illustrates an example of the offset frequency configuration forsynthetic wavelength synthetic aperture ladar operation, according toone or more aspects of the subject technology. The subject technologymay be applied to synthetic aperture ladar (SAL) and may solve thesensitivity to target motion problem inherent in SAL. As discussedabove, the amount of uncompensated target motion for a 1.5 μm SAL systemis only 8.4 nm RMS based on the SAR rule of thumb for uncompensatedphase error. This extreme sensitivity to motion would prevent successfulimage formation for targets that move or vibrate beyond this very smalltolerance. The subject technology may be configured with an arbitrary RFoffset (1006) between the frequencies of the laser signals of lasers Aand B (1002 and 1004) as depicted in FIG. 10.

The RF offset (1006) between the carrier frequencies of the two lasers(1002 and 1004) produces a synthetic wavelength which is a function ofthe center wavelengths of lasers A and B (λ_(A) and λ_(B)).

$\begin{matrix}{\lambda_{s} = \frac{\lambda_{A}\lambda_{B}}{\lambda_{B} - \lambda_{A}}} & (7)\end{matrix}$

The synthetic wavelength λ_(s) can also be expressed as a function ofthe RF offset frequency, f_(offset), as

$\begin{matrix}{\lambda_{s} = {\frac{c}{f_{offset}}.}} & (8)\end{matrix}$

Operation at a synthetic wavelength greatly reduces sensitivity totarget motion at the expense of lengthening the synthetic aperture.Without a synthetic wavelength, the allowable margin on uncompensatedphase error of 0.07 rads RMS translates to displacement in directproportion to the laser wavelength.

$\begin{matrix}{{{Allowable}\mspace{14mu}{displacment}\mspace{14mu}{RMS}} = {\frac{\left( {{.07}\mspace{14mu}{rad}} \right)}{2\pi}\frac{\lambda}{2}}} & (9)\end{matrix}$

For operation at a synthetic wavelength, the allowable displacementscales directly with the synthetic wavelength, greatly increasing themotion tolerance. To reach an objective cross range resolution of therequired length of the synthetic aperture, D_(SA), is

$\begin{matrix}{{D_{SA} = \frac{R\;\lambda}{2\Delta\; x}},} & (10)\end{matrix}$where R is the range to the target and λ is wavelength (synthetic orreal).

The penalty of a longer synthetic aperture is longer data collectiontimes. To illustrate, consider the standoff ISR imaging problem where 5cm resolution imagery is desired from an airborne platform moving at 200m/s from a range of 100 km. Assume 8 multi-look images are required forspeckle noise mitigation. Table 1 shown below contains the allowabletarget motion and data collection time for a SAR system, an opticalcarrier SAL system and two configurations of the subject invention withRF offset equal to 1 THz and 100 GHz.

TABLE 1 Carrier Collec- Ap- or λ (Real or Allowable Synthetic tionproach Offset Synthetic) Motion Aperture Time SAR  10 GHz 3 cm 170 μmRMS 30 km 20 min SAL 200 THz 1.5 μm 8.4 nm RMS 1.5 m 60 ms Inven-  1 THz300 μm 1.7 μm RMS 300 m 12 sec tion Inven- 100 GHz 3 mm 17 μm RMS 3 km 2min tion

Table 1 shows that the two configurations of the subject inventionenable a variable balance between SAR, which has good motion tolerancebut long data collection time, and SAL, which has a short datacollection time but bad motion tolerance. The RF offset in the subjecttechnology is a free parameter that may be adjusted on the fly in realtime using a broadly tunable laser to provide a variable trade betweenmotion tolerance and data collection time. The subject technologyenables operation at a high RF offset without requiring the bandwidth ofthe receiver to be equal or greater than the RF offset. This is achievedby the simultaneous angularly multiplexed coherent detection at the twowavelengths. Imaging through anisoplanatic refractive turbulence causesproblems very similar to random space-variant target motion. The abovediscussion of target motion applies equally to anisoplanatism.

FIG. 11 illustrates a diagram describing an example of a data processingsequence for synthetic aperture ladar operation, according to one ormore aspects of the subject technology. The processing sequence (502through 512) of generating the stack of E images (512) shown in FIG. 11is the same as the processing sequence (502 through 512) described withreference to FIG. 5. A stack of data, or a stack of data points, (e.g.,a stack of data as seen along an axis perpendicular to a horizontalplane) of each pixel (e.g., each pixel on the horizontal plane) withinthe stack of E images (512) provides complex-valued data that fills a 2DFourier space (1112). Each data point is collected at a differentfrequency and along track position, where the frequency is the specificvalue of the offset frequency determined by the overall offset of thetwo laser frequencies combined with the specific frequency of the lasersignal of laser B as it is being chirped. These data points are arrangedin a 2D space (1112). A 2D Fourier transform along with standardautofocus operations produces a range/cross range 2D image of the scene(1114). The stack of data in each pixel of the E image is transformedinto a 2D SAL image. The SAL images from all pixels are then stitchedtogether to produce a wide area image (not shown in FIG. 11).

FIG. 12 is a block diagram illustrating an example of a computer systemthat may be implemented utilizing the subject technology. A computersystem (1200) may represent a computer system such as the computersystem (140). In certain aspects, computer system (1200) may beimplemented using hardware or a combination of software and hardware,either in a dedicated computer or server, integrated into anotherentity, or distributed across multiple entities.

The computer system (1200) includes a bus (1208) or other communicationmechanism for communicating information, and a processor (1202) coupledwith the bus (1208) for processing information. By way of example, thecomputer system (1200) may be implemented with one or more processors(1202). The processor (1202) may include one or more processors. Theprocessor (1202) may be a general-purpose microprocessor, amicrocontroller, a Digital Signal Processor (DSP), an ApplicationSpecific Integrated Circuit (ASIC), a Field Programmable Gate Array(FPGA), a Programmable Logic Device (PLD), a controller, a statemachine, gated logic, discrete hardware components, or any othersuitable entity that can perform calculations or other manipulations ofinformation.

The computer system (1200) may include, in addition to hardware, codethat creates an execution environment for the computer program inquestion, e.g., code that constitutes processor firmware, a protocolstack, a database management system, an operating system, or acombination of one or more of them stored in a memory (1204), such as aRandom Access Memory (RAM), a flash memory, a Read Only Memory (ROM), aProgrammable Read-Only Memory (PROM), an Erasable PROM (EPROM),registers, a hard disk, a removable disk, a CD-ROM, a DVD, or any othersuitable storage device, coupled to the bus (1208) for storinginformation and instructions to be executed by the processor (1202). Theprocessor (1202) and the memory (1204) may be supplemented by, orincorporated in, special purpose logic circuitry.

The instructions may be stored in the memory (1204) and implemented inone or more computer program products, i.e., one or more modules ofcomputer program instructions encoded on a machine readable medium forexecution by, or to control the operation of, the computer system(1200). Instructions may be implemented in various computer languages.The memory (1204) may be used for storing temporary variable or otherintermediate information during execution of instructions to be executedby the processor (1202).

A computer program may be deployed to be executed on one computer or onmultiple computers that are located at one site or distributed acrossmultiple sites and interconnected by a communication network. Theprocesses and logic flows described in this specification may beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output.

The computer system (1200) may further include a data storage device(1206) such as a magnetic disk, optical disk or solid-state disk, StaticRandom Access Memory (SRAM) and Dynamic Random Access Memory (DRAM),coupled to the bus (1208) for storing information and instructions. Thecomputer system (1200) may be coupled via an input/output module (1210)to various devices. The input/output module (1210) may be anyinput/output module. The input/output module (1210) is configured toconnect to a communications module (1212). Examples of communicationsmodules (1212) include networking interface cards. In certain aspects,the input/output module (1210) is configured to connect to a pluralityof devices, such as an input device (1214) and/or an output device(1216).

The computer system (1200) may include additional components not shownin FIG. 12 or may operate with less components than those illustrated inFIG. 12. The term “machine-readable storage medium” or “computerreadable medium” as used herein refers to any medium or media thatparticipates in providing instructions or data to processor (1202) forexecution. Such a medium may be one or more media and may take manyforms, including, but not limited to, non-volatile media, and volatilemedia.

Examples of Illustrations of Subject Technology as Clauses

Various examples of aspects of the disclosure are described below asclauses. These are provided as examples, and do not limit the subjecttechnology.

A digital holography imaging system comprising:

-   -   a first laser operated with constant frequency;    -   a second laser operated with linear frequency modulation        enforced with a frequency control servo or stepped frequency        changes with equal step size;    -   flood illumination of a target with the first laser and the        second laser simultaneously;    -   a detector array;    -   a receiver optical system that focuses returned light onto the        detector array;    -   a first illumination point source wherein the first illumination        point source radiates a portion of the output from the first        laser, the location of the first illumination point source is        in-plane with the pupil of the receiver optical system, the        first illumination point source directs the illumination onto        the detector array, and the geometric relationship between the        first illumination point source and the pupil of the receiver        optical system is as illustrated in FIG. 2;    -   a second illumination point source wherein the second        illumination point source radiates a portion of the output from        the second laser, the location of the second illumination point        source is in-plane with the pupil of the receiver optical        system, the second illumination point source directs the        illumination onto the detector array, and the geometric        relationship between the second illumination point source and        the pupil of the receiver optical system is as illustrated in        FIG. 2;    -   simultaneous illumination of the focal plane array from the        first illumination point source, the second illumination point        source, and returned light from a target.

A system (or a system of any one of the clauses), including a dataprocessing method comprising:

-   -   2D Fourier transformation of each recorded frame;    -   data cropping in a first quadrant (not two quadrants) of the        Fourier domain to segregate pupil plane optical field data from        the first laser and inverse 2D Fourier transformation to obtain        a first set of complex-valued coherent images corresponding to        the first laser;    -   data cropping in a second quadrant (not two quadrants) of the        Fourier domain to segregate pupil plane optical field data from        the second laser and inverse 2D Fourier transformation to obtain        a second set of complex-valued coherent images corresponding to        the second laser;    -   conjugate product combination of each frame of the first set of        coherent images with each corresponding frame of the second set        of coherent images producing a set of conjugate product images        in which target motion induced phase errors between frames are        removed on a pixel-by-pixel basis due to shifting the phase in        the second set of coherent images by the phase measured in each        pixel of the first set of coherent images that results from the        conjugate product operation;    -   if multiple holograms are recorded at the same frequency offset,        average together the conjugate product images corresponding to a        common frequency offset to produce an improved set of conjugate        product images.

A system of any one of the clauses, including a 3D image formation dataprocessing method comprising:

-   -   a global phase compensation algorithm that removes global phase        errors between frames of the conjugate product images using a 3D        image sharpness metric maximization approach with standard        nonlinear optimization algorithms;    -   a 1D Fourier transformation through the frames producing a 3D        image;    -   if multiple 3D images are collected, combine them by averaging        the intensities;    -   a peak estimation algorithm wherein interpolated range points        are calculated using Fourier domain zeropadding, a parabola is        fit to the peak value and 2 adjacent values, and the summit of        the parabola is reported as the range report;    -   a peak selection algorithm wherein the range report in each        pixel is compared to the median the range reports in a        neighborhood of pixels, and if the range report exceeds the        median of the neighbors by an amount greater than the standard        deviation of the neighbors, the report is rejected and the test        is repeated for the next highest peak until a report is found        that satisfies the criteria and if none are found then the        highest peak is reported.

A system of any one of the clauses, wherein the imaging system isoperated on a moving platform in a translational synthetic apertureimaging configuration, or the imaging system observes a rotating targetin an inverse synthetic aperture imaging configuration (where targetrotation provides a diversity of look angles), or some combination ofboth configurations.

A system of any one of the clauses, including a synthetic aperture ladarimage formation data processing method comprising:

-   -   two dimensional arrangement the data in each pixel of the        conjugate product images wherein the placement of the data in        the 2D space is done according to the specific values of the        offset frequency and the along track position at the time the        data was recorded;    -   application of standard autofocus algorithms as appropriate;    -   2D Fourier transformation to produce a 2D SAL image from the        data in a single pixel of the conjugate product image;    -   formation of individual 2D SAL images for all pixels of the        conjugate product image set according to the above steps;    -   incoherently average together multi-look SAL images as        appropriate, where each multi-look image is obtained at a        different along-track position to produce independent        realizations of speckle noise;    -   stitch together all the individual 2D SAL images into a larger        wide area SAL image.

A system of any one of the clauses, wherein the detector array is placedin the pupil plane or any intermediate plane of the receiver opticalsystem

A system of any one of the clauses, further comprising one or more laseramplifiers that amplify the power of the first laser and/or the secondlaser.

A system of any one of the clauses, wherein the Fourier tranformationsare computed using the FFT algorithm.

A system of any one of the clauses, wherein the lasers and/or laseramplifiers are operated in continuous wave mode.

A system of any one of the clauses, wherein the lasers and/or the laseramplifiers are operated in pulsed mode where the input to the amplifiersare pulsed using a first amplitude modulator and the first and thesecond illumination point source are operated in pulsed mode where thepulse is generated using a second and third amplitude modulator andtiming of the first and second illumination pulses are arranged tooverlap the time the returned single pulse is received.

A system (or a system of any one of the clauses), having motioncompensation by using data from one laser as a pixel-by-pixel phasereference for the other.

A system (or a system of any one of the clauses), having simultaneousdual wavelength operation where the frequency separation is linearlytuned.

A system (or a system of any one of the clauses), having pupil planeangular multiplexing of the two wavelengths in a coherent digitalholography receiver.

A system (or a system of any one of the clauses), having operation withan RF offset between the two lasers to produce a synthetic wavelengththat reduces target motion sensitivity for synthetic aperture ladar.

Other Descriptions

In one aspect, clauses herein, if any, may depend from any one of theindependent clauses or any one of the dependent clauses. In one aspect,any clause (e.g., dependent or independent clauses) may be combined withany other one or more clauses (e.g., dependent or independent clauses).In one aspect, a claim may be amended to depend from one or more otherclaims or may be amended to be combined with one or more other claims.In one aspect, a claim may be amended to include some or all of thewords (e.g., steps, operations, means or components) recited in one ormore clauses, one or more sentences, one or more phrases, one or moreparagraphs, or one or more claims. In one aspect, the subject technologymay be implemented without utilizing some of the components, elements,functions or operations described herein. In one aspect, the subjecttechnology may be implemented utilizing additional components, elements,functions or operations.

In one aspect, any methods, instructions, code, means, logic,components, blocks, modules and the like (e.g., software or hardware)described or claimed herein can be represented in drawings (e.g., flowcharts, block diagrams), such drawings (regardless of whether explicitlyshown or not) are expressly incorporated herein by reference, and suchdrawings (if not yet explicitly shown) can be added to the disclosurewithout constituting new matter. For brevity, some (but not necessarilyall) of the clauses/descriptions/claims are explicitly represented indrawings, but any of the clauses/descriptions/claims can be representedin drawings in a manner similar to those drawings explicitly shown. Forexample, a flow chart can be drawn for any of the clauses, sentences orclaims for a method such that each operation or step is connected to thenext operation or step by an arrow(s)/line(s). In another example, ablock diagram can be drawn for any of the clauses, sentences or claimshaving means-for elements (e.g., means for performing an action) suchthat each means-for element can be represented as a module for element(e.g., a module for performing an action).

Those of skill in the art would appreciate that items such as thevarious illustrative blocks, modules, elements, components, methods,operations, steps, and algorithms described herein may be implemented ashardware, computer software, or a combination of both.

To illustrate the interchangeability of hardware and software, itemssuch as the various illustrative blocks, modules, elements, components,methods, operations, steps, and algorithms have been described generallyin terms of their functionality. Whether such functionality isimplemented as hardware or software depends upon the particularapplication and design constraints imposed on the overall system.Skilled artisans may implement the described functionality in varyingways for each particular application.

A reference to an element in the singular is not intended to mean oneand only one unless specifically so stated, but rather one or more. Forexample, “a” module may refer to one or more modules. An elementproceeded by “a,” “an,” “the,” or “said” does not, without furtherconstraints, preclude the existence of additional same elements.

Unless specifically stated otherwise, the term some refers to one ormore. Pronouns in the masculine (e.g., his) include the feminine andneuter gender (e.g., her and its) and vice versa. Headings andsubheadings, if any, are used for convenience only and do not limit theinvention. The word exemplary is used to mean serving as an example orillustration. Like components are labeled with identical element numbersfor ease of understanding.

To the extent that the term include, have, or the like is used, suchterm is intended to be inclusive in a manner similar to the termcomprise as comprise is interpreted when employed as a transitional wordin a claim. Relational terms such as first and second and the like maybe used to distinguish one entity or action from another withoutnecessarily requiring or implying any actual such relationship or orderbetween such entities or actions.

Phrases such as an aspect, the aspect, another aspect, some aspects, oneor more aspects, an implementation, the implementation, anotherimplementation, some implementations, one or more implementations, anembodiment, the embodiment, another embodiment, some embodiments, one ormore embodiments, a configuration, the configuration, anotherconfiguration, some configurations, one or more configurations, thesubject technology, the disclosure, the present disclosure, othervariations thereof and alike are for convenience and do not imply that adisclosure relating to such phrase(s) is essential to the subjecttechnology or that such disclosure applies to all configurations of thesubject technology. A disclosure relating to such phrase(s) may apply toall configurations, or one or more configurations. A disclosure relatingto such phrase(s) may provide one or more examples. A phrase such as anaspect or some aspects may refer to one or more aspects and vice versa,and this applies similarly to other foregoing phrases.

In one aspect, unless otherwise stated, all measurements, values,ratings, positions, magnitudes, sizes, and other specifications that areset forth in this specification, including in the claims that follow,are approximate, not exact. In one aspect, they are intended to have areasonable range that is consistent with the functions to which theyrelate and with what is customary in the art to which they pertain. Inone aspect, some of the dimensions are for clarity of presentation andare not to scale.

In one aspect, a term coupled or the like may refer to being directlycoupled. In another aspect, a term coupled or the like may refer tobeing indirectly coupled.

Terms such as top, bottom, front, rear, side, horizontal, vertical, andthe like refer to an arbitrary frame of reference, rather than to theordinary gravitational frame of reference. Thus, such a term may extendupwardly, downwardly, diagonally, or horizontally in a gravitationalframe of reference.

A phrase “at least one of” preceding a series of items, with the terms“and” or “or” to separate any of the items, modifies the list as awhole, rather than each member of the list. The phrase “at least one of”does not require selection of at least one item; rather, the phraseallows a meaning that includes at least one of any one of the items,and/or at least one of any combination of the items, and/or at least oneof each of the items. By way of example, each of the phrases “at leastone of A, B, and C” or “at least one of A, B, or C” refers to only A,only B, or only C; any combination of A, B, and C; and/or at least oneof each of A, B, and C.

Various items may be arranged differently (e.g., arranged in a differentorder, or partitioned in a different way) all without departing from thescope of the subject technology.

It is understood that the specific order or hierarchy of steps,operations or processes disclosed is an illustration of exemplaryapproaches. Unless explicitly stated otherwise, it is understood thatthe specific order or hierarchy of steps, operations or processes may berearranged. Some of the steps, operations or processes may be performedsimultaneously. The accompanying method claims, if any, present elementsof the various steps, operations or processes in a sample order, and arenot meant to be limited to the specific order or hierarchy presented.

The disclosure is provided to enable any person skilled in the art topractice the various aspects described herein. In some instances,well-known structures and components are shown in block diagram form inorder to avoid obscuring the concepts of the subject technology. Thedisclosure provides various examples of the subject technology, and thesubject technology is not limited to these examples. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects.

All structural and functional equivalents to the elements of the variousaspects described throughout this disclosure that are known or latercome to be known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe claims. Moreover, nothing disclosed herein is intended to bededicated to the public regardless of whether such disclosure isexplicitly recited in the claims. No claim element is to be construedunder the provisions of 35 U.S.C. §112, sixth paragraph, unless theelement is expressly recited using a phrase means for or, in the case ofa method claim, the element is recited using the phrase step for.

The title, background, summary, brief description of the drawings,abstract, and appended drawings are hereby incorporated into thedisclosure and are provided as illustrative examples of the disclosure,not as restrictive descriptions. It is submitted with the understandingthat they will not be used to limit the scope or meaning of the claims.In addition, in the detailed description, it can be seen that thedescription provides illustrative examples and the various features aregrouped together in various embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed subject matter requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter lies in less than allfeatures of a single disclosed configuration or operation. The followingclaims are hereby incorporated into the detailed description, with eachclaim standing on its own as a separately claimed subject matter.

The claims are not intended to be limited to the aspects describedherein, but are to be accorded the full scope consistent with thelanguage claims and to encompass all legal equivalents. Notwithstanding,none of the claims are intended to embrace subject matter that fails tosatisfy the requirements of the applicable patent law, nor should theybe interpreted in such a way.

What is claimed is:
 1. A holography imaging system, comprising: a firstlaser configured to generate an output having a constant frequency; asecond laser configured to generate an output having a non-constantfrequency; a transmitter optical system configured to illuminate atarget simultaneously using a first portion of the output of the firstlaser and a first portion of the output of the second laser; a detectorarray; a receiver optical system configured to receive a returned lightfrom the illuminated target and focus the returned light onto thedetector array; a first illumination point source configured to radiatea second portion of the output of the first laser, wherein a location ofthe first illumination point source is in-plane with a pupil of thereceiver optical system, and the first illumination point source isconfigured to direct the second portion of the output of the first laseronto the detector array; and a second illumination point sourceconfigured to radiate a second portion of the output of the secondlaser, wherein a location of the second illumination point source isin-plane with the pupil of the receiver optical system, and the secondillumination point source is configured to direct the second portion ofthe output of the second laser onto the detector array, wherein theholography imaging system is configured to detect simultaneouslyholograms formed on the detector array based on the returned light, thesecond portion of the output of the first laser, and the second portionof the output of the second laser.
 2. The holography imaging system ofclaim 1, wherein the returned light comprises a first returned light anda second returned light, wherein the first returned light is based onthe first portion of the output of the first laser, wherein the secondreturned light is based on the first portion of the output of the secondlaser, wherein the holograms comprise a first hologram and a secondhologram, wherein the first hologram is based on the first returnedlight and the second portion of the output of the first laser, whereinthe second hologram is based on the second returned light and the secondportion of the output of the second laser, wherein the first portion ofthe output of the first laser comprises a first wavelength, wherein thesecond portion of the output of the first laser comprises the firstwavelength, wherein the first returned light comprises the firstwavelength, wherein the first hologram comprises the first wavelength,wherein the first portion of the output of the second laser comprises asecond wavelength, wherein the second portion of the output of thesecond laser comprises the second wavelength, wherein the secondreturned light comprises the second wavelength, wherein the secondhologram comprises the second wavelength, wherein the first wavelengthis different from the second wavelength, and wherein the holographyimaging system is configured to detect the first hologram and the secondhologram simultaneously.
 3. The holography imaging system of claim 1,wherein the second laser is configured to operate with linear frequencymodulation enforced with a frequency control servo.
 4. The holographyimaging system of claim 1, wherein the second laser is configured tooperate with stepped frequency changes with an equal step size.
 5. Theholography imaging system of claim 1, wherein the first and secondillumination point sources are in an exit pupil plane of the receiveroptical system, wherein the receiver optical system has the exit pupilplane, and a (x,y) coordinate system is defined in the exit pupil plane,wherein an exit pupil is centered at coordinates (0,0), wherein thefirst illumination point source is located at coordinates$\left( {{- \frac{Q_{1}D}{4}},{- \frac{Q_{1}D}{4}}} \right),$ wherein Dis an exit pupil diameter, and Q₁ is a sampling ratio, defined as${Q_{1} = \frac{z_{i}\lambda_{1}}{D\; p}},$ wherein z_(i) is a distancebetween the exit pupil and the detector array, λ₁ is a wavelength of thefirst laser, and p is a pixel pitch of the detector array.
 6. Theholography imaging system of claim 5, wherein the second illuminationpoint source is located at coordinates$\left( {\frac{Q_{2}D}{4},{- \frac{Q_{2}D}{4}}} \right),$ wherein D isthe exit pupil diameter, and Q₂ is a sampling ratio, defined as${Q_{2} = \frac{z_{i}\lambda_{2}}{D\; p}},$ wherein λ₂ is a wavelengthof the second laser, and p is the pixel pitch of the detector array. 7.The holography imaging system of claim 1, wherein the detector array islocated in a pupil plane of the receiver optical system.
 8. Theholography imaging system of claim 1, wherein the detector array islocated in an intermediate plane of the receiver optical system.
 9. Theholography imaging system of claim 1, comprising one or more laseramplifiers configured to amplify at least one of a power of the firstlaser or a power of the second laser.
 10. The holography imaging systemof claim 1, wherein at least one of the first laser, the second laser,or one or more laser amplifiers is configured to operate in a continuouswave mode.
 11. The holography imaging system of claim 9, wherein atleast one of the first laser, the second laser, or the one or more laseramplifiers is configured to operate in a continuous wave mode.
 12. Theholography imaging system of claim 1, comprising one or more amplitudemodulators, wherein the holography imagining system is configured tooperate in a pulse mode, wherein the first portion of the output of thefirst laser comprises a first pulse, wherein the first portion of theoutput of the second laser comprises a second pulse, wherein thereturned light comprises a returned pulse, wherein the returned pulse isbased on the first pulse and the second pulse, wherein the secondportion of the output of the first laser comprises a third pulse,wherein the second portion of the output of the second laser comprises afourth pulse, wherein the holography imaging system is configured toilluminate the target with the first and second pulses simultaneously,wherein the holography imaging system is configured to detectsimultaneously the holograms formed on the detector array based on thereturned pulse, the third pulse and the fourth pulse, and wherein apulse duration of the returned pulse is shorter than a coherence time ofthe returned pulse.
 13. The holography imaging system of claim 9,comprising one or more amplitude modulators, wherein the holographyimagining system is configured to operate in a pulse mode, wherein thefirst portion of the output of the first laser comprises a first pulse,wherein the first portion of the output of the second laser comprises asecond pulse, wherein the returned light comprises a returned pulse,wherein the returned pulse is based on the first pulse and the secondpulse, wherein the second portion of the output of the first lasercomprises a third pulse, wherein the second portion of the output of thesecond laser comprises a fourth pulse, wherein the holography imagingsystem is configured to illuminate the target with the first and secondpulses simultaneously, wherein the holography imaging system isconfigured to detect simultaneously the holograms formed on the detectorarray based on the returned pulse, the third pulse and the fourth pulse,and wherein a pulse duration of the returned pulse is shorter than acoherence time of the returned pulse.
 14. The holography imaging systemof claim 1, comprising: one or more memories comprising instructionsstored therein; and a processor configured to execute instructions to:record the holograms formed on the detector array, transform therecorded holograms using two-dimensional Fourier transformation intotransformed data in a Fourier domain; perform data cropping on thetransformed data in a first portion of the Fourier domain to segregatefirst pupil plane optical field data associated with the first laser,and perform inverse two-dimensional Fourier transformation on the firstpupil plane optical field data to obtain a first set of complex-valuedcoherent images associated with the first laser; perform data croppingon the transformed data in a second portion of the Fourier domain tosegregate second pupil plane optical field data associated with thesecond laser, and perform inverse two-dimensional Fourier transformationon the second pupil plane optical field data to obtain a second set ofcomplex-valued coherent images associated with the second laser, whereinthe second portion is different from the first portion; and performconjugate product combination of each frame of the first set ofcomplex-valued coherent images with each corresponding frame of thesecond set of complex-valued coherent images to produce a set ofconjugate product images.
 15. The holography imaging system of claim 14,wherein the instructions to perform conjugate product combinationcomprises removing target motion induced phase errors between theframes, of the first set of complex-valued coherent images and thesecond set of complex-valued coherent images, on a pixel-by-pixel basisby shifting a phase in each pixel of the frames of the second set ofcomplex-valued coherent images using a phase measured in a correspondingpixel of the frames of the first set of complex-valued coherent images.16. The holography imaging system of claim 14, wherein the processor isconfigured to execute instructions to average together frames of the setof conjugate product images, wherein each frame in the set of conjugateproduct images to be averaged is collected at a common offset frequencybetween the first and second lasers, to produce a second set ofconjugate product images.
 17. The holography imaging system of claim 14,wherein the processor is configured to execute instructions to perform athree-dimensional image formation data processing operation, wherein thethree-dimensional image formation data processing operation comprises: aglobal phase compensation operation to remove global phase errorsbetween images of the set of conjugate product images to produce a thirdset of conjugate product images; a one-dimensional Fouriertransformation operation performed on each lateral element of the thirdset of conjugate product images to produce a 3D image; a peak estimationoperation to compute a range for each lateral element in the 3D image,comprising Fourier domain zero-padding to compute interpolated rangepoints and fitting a parabola to a peak interpolated point along withtwo adjacent points, wherein a summit of the parabola is a range report;a peak selection operation wherein (a) a lateral element is selected forprocessing, (b) a neighborhood of lateral element range reports isdefined surrounding the selected lateral element, (c) a median of theneighborhood range reports is computed, (d) a standard deviation of theneighborhood range reports is computed, and (e) the range report of theselected lateral element is compared to the median, wherein if adifference between the range report and the median is less than or equalto the standard deviation, accepting the range report, and if not,rejecting the range report and repeating the above test of (b) through(e) for the next highest peak in the selected lateral element, and thenrepeating the above process of (a) through (e) for each lateral elementin the 3D image.
 18. The holography imaging system of claim 14, whereinthe holography imaging system is configured to operate on a movingplatform in a translational synthetic aperture imaging configuration,the holography imaging system is configured to observe a rotating targetin an inverse synthetic aperture imaging configuration, wherein targetrotation provides a diversity of look angles, or the holography imagingsystem is configured to operate using a combination of both of theconfigurations.
 19. The holography imaging system of claim 14, whereinthe processor is configured to execute instructions to perform asynthetic aperture ladar image formation data processing operation,wherein the synthetic aperture ladar image formation data processingoperation comprises: for each lateral element of the set of conjugateproduct images, arranging data in the each lateral element into a twodimensional space, based on a value of an offset frequency and an alongtrack position, wherein the value of an offset frequency is determinedbased on a difference between the first and second laser frequencies,wherein a value of the along track position is determined based on arelative position of a platform along the platform's trajectory and thetarget; applying an autofocus operation on at least some of the data inthe two dimensional space; for each lateral element of the set ofconjugate product images, applying two-dimensional Fouriertransformation on the data in the two dimensional space corresponding tothe lateral element, to produce a two-dimensional synthetic apertureladar (2D SAL) image corresponding to each lateral element of the set ofconjugate product images; incoherently averaging the 2D SAL images,wherein the 2D SAL images are multi-look images; and stitching togetherall of the 2D SAL images into a larger wide area SAL image.
 20. Theholography imaging system of claim 19, wherein the incoherentlyaveraging the 2D SAL images comprises averaging together the 2D SALimages, wherein each of the 2D SAL images to be averaged is collectedwith independent speckle realizations.